Coaxial cable with inner and outer sodium conductors



G. J. THOMPSON ETAL 3,507,976

COAXIAL CABLE WITH INNER AND OUTER SODIUM CONDUCTORS April 21, 1970 Filed March l0, 1969 IVEV United States Patent O U.S. Cl. 174-74 3 Claims ABSTRACT F THE DISCLOSURE A coaxial cable in which at least the outer conductor is of sodium terminated by polyethylene sealing rings through which extend conductive braids in electrical contact with the ends of the sodium conductor to serve as a lower loss radio frequency cable and one having more constant electrical characteristics when flexed.

The invention described herein may be manufactured and used by or for the Govrenment of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION Conventional exible radio frequency coaxial cable consists of a center conductor of solid or silverplated-stranded copper and a noble-metal-plated cylindrical braid outer conductor that surrounds the dielectric material over the inner conductor. Attenuation in conventional coaxial cable s in large part due to the losses of the outer conductor. In the outer conductor, conduction paths include series of contacts which are braid wire Crossovers. Though the losses of an outer conductor of braid construction cannot be as low as that of a continuous cylinder of the same metal and of the same dimensions, a rigid structure conductor is mechanically unfeasible. Also in conventional coaxial cable, the loss changes when the cable is mechanically disturbed. Apparently when the cable is disturbed, ilexure or change in tension tends to shift contact Crossovers among the braid wires and to modify the distribution of the braid wires around the inner conductor. The mechanical disturbance may be due to gusts of wind, vibration of support structures, or other unavoidable movement.

An object of this invention is to provide a more stable and lower loss radio frequency coaxial cable.

Another object of the invention is to provide a ilexble radio frequency coaxial cable having more stable electrical parameters during use and lower loss than c0- axial cables having noble-metal-plated stranded and braided outer conductors. p

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a longitudinal section of an embodiment of this invention fabricated for laboratory purposes, and

FIG. 2 is a cross section of a coaxial cable wherein both conductors are sodium.

In the embodiment shown in FIG. l, the' insulated center conductor is that of a commercial coaxial cable e.g. RG9/U. At each end of the insulated center conductor 10 a short length of outer conductor braid 12, 14 is retained. Close-fitting polyethylene rings 16, 18 are located over the braid segments and a length of irradiated polyethylene tubing slightly shorter than the insulated conductor 10 is slid over the rings 16, 18 leaving a fraction of the length of the braid exposed at each 3,507,976 Patented Apr. 21, 1970 end. Clamps 22, 24 grip the ends of the insulated center conductor and hold it taut horizontally. Then hose clamps 26, `28 around the tubing portions surrounding the rings 16, 18 are tightened. Holes 30, 32 are formed in opposite ends of the tubing 20 inwardly of the rings 16, 18 through the topmost part of the tubing. A tapelike heating element not shown is wrapped snugly around the tubing and is connected to a power supply to heat the tubing to a temperature between the melting point of sodium and the softening point of irradiated polyethylene. Then a supply of sodium is fused at 208 F. under cover of high flash point oil. Molten sodium is delivered through an irradiated polyethylene tube 34 to one of the holes in the outside insulation tubing 20 to fill the volume between the outside tubing and the insulated inner conductor 10 while the other hole serves as a vent. The heating element prevents premature solidification of molten sodium discharged into the outside insulating tubing 20. Because of the highly reactive character of sodium the method described provides for introducing sodium into the outside tubing with minimum exposure to air inside the tubing and which is dislpaced by the sodium. After the tubing 20 is filled with sodium, the holes are sealed with molten polyethylene and the heater tape is deenergized.

The braid makes good electrical contact with the sodium and the fraction of braid that extends outwardly beyond the rings .16, 18 is intended for making connection to other sections of coaxial cable or to a terminal.

rConventional extrusion techniques and equipments are employed to form continuous long lengths of coaxial cable with a sodium outer conductor; provision is made to exclude oxygen and moisture from the sodium by carrying out the procedure in an inert atmosphere in a suitable chamber. A concentric polyethylene insulated conductor is continuously drawn through a lead-press or through a header into which liquid sodium is pumped to overlay the polyethylene with a coaxial layer, viz, 20 mils, of sodium. As the cable is drawn out of the header the sodium layer cools and then is passed through a polyethylene insulation extruder, a cooling bath, puller reels and onto a takeup reel. No provision is needed for monitoring nor for controlling concentricity of the outer surface of the sodium layer and the outer polyethylene layer. After the outer jacket of polyethylene is applied, the cable can enter into the atmosphere for completion of the process.

The inner conductor of the invention described may be noble-metal-plated solid or stranded copper. In an alternative construction shown in FIG. 2, the inner conductor is sodium. The sodium core 36 is extruded as a solid cylinder and is jacketed with irradiated polyethylene 38. The thickness of the polyethylene jacket and the concentricity is carefully maintained. The outer conductor 40 is sodium and the outer insulation 42 is polyethylene as described above.

There is savings in weight where sodium is substituted for copper to have approximately the same conductivity as the copper. Coaxial cable having a sodium outer conductor can be fabricated at higher speeds than coaxial cable having noble metal plated braid for the outer conductor since braiding is slower than extrusion and can be fabricated at lower cost. Extruded sodium as outer conductor has greater uniformity of quality than copper braid outer conductor. A coaxial cable with a sodium outer conductor has greater ability to withstand repeated flexing, high external pressures which accelerate the destructive abrasion of the braids and which provides increased pliability, an important factor in handling and installation. An extruded sodium outer conductor has no impedance discontinuities comparable to the Crossovers of braided wire and hence such coaxial cable has aninherently lower voltage standing wave ratio, is not likely to launch undesired modes of propagation, has greater stability of transmission characteristics, and transmits satisfactorily at higher frequency. Additionally a Coaxial cable having an extruded sodium outer conductor performs better than conventional cable under repeated flexing at lower temperature, Viz, 50 C.

We claim: 11. A length of flexible coaxial cable comprising: an inner conductor surrounded by an inner continuous [acket of insulating material, a a continuous cylindrical outer conductor of sodium overlaying said cylindrical jacket in surface-to-surface engagement and surrounded by an outer cylin- 1V drical jacket of insulating material, iV

a ring member at each end of the sodium conductor and surrounded by the outer jacket of insulating material and sealing in the sodium conductor, and a metal conductor extending through each ring member and in conductive-engagement with the sodium conductor for connection to a circuit.

2. A length of flexible coaxial cable as dettined in claim 1 wherein said inner conductor is a continuous sodium core. 5

3. A length of yiiexible coaxial cable as defined in claim I1 further comprising a'clamp gripping the outer jacket of insulating material around each ring member.

LARAMIE E. ASK'IN, Primary Examiner U.S. C1. X.R. 

